This invention relates to a magnetic recording medium, such as a magnetic tape. More particularly, it relates to a particulate magnetic recording medium which is less prone to suffer from reduction in output and maintains stable running properties when used or stored under various environmental conditions.
Magnetic recording technology has enjoyed wide-scale adoption in various fields including video, audio, and computers because of its unparalleled characteristics such that a recording medium can be used repeatedly, signals are converted easily, making it possible to construct a system with peripheral equipment, and signals can be corrected easily.
In order to meet the outstanding demands for equipment size reduction, improvement on write and read signal quality, extension of recording time, increase of recording capacity, and the like, it has always been required to develop recording media with higher recording density, higher reliability, and improved durability.
In the audio and video fields, for example, a digital recording system which realizes improvement on sound and image qualities has been put to practical use, and an image recording system coping with high-definition TV broadcasting has been developed. These trends have boosted the demand for magnetic recording media which are capable of recording and reproducing shorter wavelength signals than with conventional systems and maintaining excellent reliability and durability even if the medium""s running speed relative to a recording head increases. For computer applications, too, it has been keenly demanded to develop a large capacity digital recording medium capable of storing an ever-increasing amount of data.
Various approaches have been proposed to increase the recording density of particulate magnetic recording media, such as replacement of conventionally used magnetic iron oxide powder with iron or iron-based alloy magnetic powder, improvement on magnetic characteristics of magnetic powder by particle size reduction or by improving powder packing and orientation, improvement on ferromagnetic powder dispersibility, and improvement on surface properties of the magnetic layer.
For example, use of a hexagonal ferrite powder as ferromagnetic powder is proposed to obtain increased magnetic characteristics as disclosed in JP-A-58-122623, JP-a-61-74137, JP-B-62-49656, JP-B-60-50323, and U.S. Pat. Nos. 4,629,653, 4,666,770, and 4,543,198.
Addition of various surface active agents is suggested to increase dispersibility of ferromagnetic powder as disclosed in JP-A-52-156606, JP-A-53-15803, and JP-A-53-116114. For the same purpose addition of various reactive coupling agents is taught in JP-A-49-59608, JP-A-56-58135, and JP-B-62-28489. Surface treatments on an applied and dried magnetic layer have also been proposed to improve the surface properties of the magnetic layer as described in JP-B-60-44725.
A number of processes are available for producing hexagonal ferrite magnetic powders. Known processes include (a) a coprecipitation process comprising bringing into contact a metal ion aqueous solution containing iron chloride, an alkaline earth metal salt and, if necessary, a chloride of a doping element with an alkali (e.g., NaOH) solution to coprecipitate metal ions, washing the precipitate with water, drying the precipitate, and crystallizing the precipitate at a high temperature, (b) a hydrothermal process comprising crystallizing metal ions from the metal ion aqueous solution used in the process (a) in a high-temperature high-pressure container and, if necessary, heating the crystals in high temperatures (see JP-A-56-160328), (c) a flux method comprising crystallizing compounds containing iron and an alkaline earth metal at high temperatures in the presence of a flux (e.g., NaCl or BaCl2) and removing the flux from the crystals, and (d) a process by controlled crystallization of glass which comprises blending a glass-forming oxide (e.g., BaO, B2O3 or SiO2), an iron compound, a barium compound and, if desired, a compound of a doping element, melting the blend, rapidly cooling the melt into an amorphous solid (glass), re-heating the solid in high temperatures for crystallization, and removing the glass-forming oxide (see JP-A-56-67904). JP-A-60-240107 and JP-A-3-78209 teach that barium ions dissolved from magnetic powder impair wear resistance of the magnetic layer, resulting in reduction of reliability for output, running stability, and durability and that this problem is solved by treating the magnetic layer with water containing sulfate ions or carbonate ions.
As the demands for equipment size reduction, improvement on write and read signal quality, extension of recording time, and increase of recording capacity are being achieved, the diversity of the environment in which magnetic recording media are used are increasing. It has now come to be necessary for magnetic recording media to show as stable running properties as in usual applications even when used or stored in various environments. A magnetic recording medium having at least two layers on a support, i.e., anon-magnetic layer mainly comprising non-magnetic powder and a binder as a lower layer and a magnetic layer mainly comprising ferromagnetic powder and a binder as an upper layer, exhibits high performance because it has in principle low self-magnetization loss and also has small surface roughness, namely small spacing loss. However, such a multilayer magnetic recording medium has turned out to suffer from the following problems when stored under high-temperature and high-humidity conditions depending on the surface properties of the hexagonal ferrite powder used in the upper magnetic layer and the non-magnetic powder used in the lower non-magnetic layer and impurities of these powders. That is, it shows a reduced output in electromagnetic measurement and an increased frictional coefficient in a running test after storage under high-temperature and high-humidity conditions. In an extreme case the medium can cling to a magnetic head to stop running.
An object of the present invention is to provide a magnetic recording medium which is less prone to suffer from reduction in output and keeps excellent running properties when used or stored under various environmental conditions.
The present invention provides a magnetic recording medium comprising a support and at least a magnetic layer containing a hexagonal ferrite powder and a binder, wherein the hexagonal ferrite powder is of magnetoplumbite type and has an average diameter of 15 to 35 nm and an alkaline earth element to iron ratio of 0.10 to 0.15 in terms of peak intensity ratio analyzed by electron spectroscopy for chemical analysis (hereinafter abbreviated as ESCA).